【Oshi no Ko】Aqua and Ruby: A Forbidden Bond Beyond Siblings? A Complete Analysis of Their Relationship from Past Lives to the Ending

Introduction: The Twin Stars at the Heart of the Story

In the phenomenal anime/manga "Oshi no Ko," the twin siblings Hoshino Aquamarine (Aqua) and Hoshino Ruby are undoubtedly the core that drives the entire story forward. Their relationship is far more complex than surface-level sibling affection, intertwining past-life promises, present-life blood ties, shared trauma, and vastly different paths of revenge. This unique bond is both the starting point of the story and what leads readers toward its controversial and tragic conclusion.

The Birth of the Twin Stars: A Past-Life Promise and a Present-Life Reunion

The relationship between Aqua and Ruby must be traced back to their past lives. Aqua's past life was the rural obstetrician-gynecologist "Amamiya Gorou," and Ruby's past life was his patient, the idol-obsessed girl "Tendouji Sarina," who died young from illness. In Sarina's final days, Gorou was her only confidant and emotional support; she naively confessed her love to the doctor, and Gorou promised her that when she grew up and became an idol, he would definitely become her number one fan.

This unfulfilled promise continued in an unexpected way after they were both reborn as children of the idol "Hoshino Ai." Although they both retained memories of their past lives, for a long time, they did not realize that the other was that most special person from their previous existence.

A Shared Scar, Diverging Paths

The fate of these siblings was completely changed the moment their mother, Hoshino Ai, was assassinated before their eyes. This shared scar, however, set them on entirely different paths.

• Aqua's Path of Darkness: Having witnessed his mother's brutal death firsthand, Aqua's heart was consumed by the flames of revenge. His life's goal narrowed down to finding and punishing the mastermind behind it—their biological father, Kamiki Hikaru. To this end, he did not hesitate to step into the dark side of the entertainment industry, viewing everyone around him as pawns for his revenge, and harboring an extreme protective desire towards Ruby, attempting to prevent her from repeating their mother's tragedy.
• Ruby's Path to Stardom: In contrast, Ruby's initial dream was more pure; she aspired to become an idol shining like her mother. However, after learning the truth about the revenge, especially upon discovering that the doctor she revered in her past life was also harmed by the same person, she too became driven by hatred and once fell into darkness.

The two had a serious disagreement over methods of revenge and whether their mother's background should be made public, causing their relationship to plummet to a freezing point.